1. Field of the Invention
The present invention relates to an apparatus for lapping sliders, and particularly to a mechanism for pushing a bar against a lapping plate.
2. Description of the Related Art
Sliders used in hard disk drives are fabricated through a wafer process in which read elements and write elements are formed, a process for dicing the wafer into blocks or bars, a lapping process for forming a predetermined air bearing surface, and so on. The lapping process usually consists of two or three separate lapping processes.
First, a rough lapping process, which may be omitted, is performed in order to improve efficiency in the subsequent element height forming lapping process. In this rough lapping process, a block or a bar having a number of elements that are to be formed into sliders (hereinafter, simply referred to as “elements”) formed thereon is lapped until the read element height reaches a target value thereof. The term “read element height” as used herein means a length (depth) of a read element that is measured in a direction that is perpendicular to the air bearing surface of an MR (Magneto Resistive) element, and the read element height plays an important role in achieving preferable properties, such as an MR ratio.
Next, in order to accurately form the read element height, a second lapping process called an element height forming lapping process is performed. This lapping process is also called a height adjustment lapping process. Accurate formation of the read element height is significantly important, and a lapping method using resistance elements, such as RLG (Resistance Lapping Guide), is known. The resistance elements are formed in advance between the MR elements in a wafer process, and each resistance element is electrically connected, at both ends thereof, to pads, which are formed on a surface of a bar that is other than the lapping surface of the bar, via inside of the elements. During lapping, electric resistance of the resistance elements is measured via the pads. The resistance elements are lapped together with the MR elements, and thereby the electric resistance of the resistance elements is increased as lapping progresses. Thus, by obtaining, in advance, the relationship between the amount in which the elements are lapped and the electric resistance and by lapping the elements while monitoring the electric resistance of the resistance elements, it is possible to indirectly estimate the lapping amount of the elements during lapping.
However, when a plurality of elements, which are formed in a bar, are simultaneously lapped using the above described method, it is not possible to completely prevent variation in the lapping amount among the elements during lapping. Recently, a technology has been disclosed to reduce the variation in the lapping amount during lapping by using a plurality of pressing cylinders for individual elements and thereby applying optimum pressing force to each element (see Japanese Patent Laid-Open Publication No. 2002-157723).
The final lapping process is a so-called surface finishing lapping process, which is often called a touch lap process. In the surface finishing lapping process, a mirror finished lapping plate is used to lap the air bearing surface. The surface finishing lapping process removes scratches and the like on the air bearing surface so that smoothness of the air bearing surface can be improved. In this process, a convex shape called a crown is simultaneously formed on the air bearing surface, which is important for flying properties of a slider. In the surface finishing lapping process, the lapping amount itself is not monitored because the lapping amount is small and the pressing force is limited. Lapping is completed when a certain period of time of lapping lapses based on a lapping rate which has been estimated in advance. As means for applying pressing force, Japanese Patent Laid-Open Publication No. 2002-157723 discloses a method for applying optimum pressing force to each element by using a plurality of pressing cylinders, as carried out in the element height forming lapping process. Also, a simpler method is disclosed in Japanese Patent Laid-Open Publication No. 249714/98, in which a weight is put on a lapping head that holds elements.
When a pressing cylinder is employed, an air actuation type is typically used. FIG. 1 is a conceptual sectional view illustrating a cylinder section that generates pressing force. Piston 72 is slidably mounted in cylinder 71, and pusher 6 is connected to the end of piston 72. Accordingly, the pushing force of pusher 6 can be controlled by controlling movement of piston 72. Air tube 10 for supplying air into cylinder 71 is connected to one end of the cylinder. A plurality of pushers 6 are provided in the longitudinal direction of a bar, and the pushing force of each pusher 6 can be individually controlled by adjusting the amount of air that is supplied into cylinder 71 and thereby controlling pressure in cylinder 71. Piston 72 may be integrated with pusher 6.
It is desirable that the bar be pressed with force that is as uniform as possible while being lapped. If the bar is subjected to large pushing force locally, only the portion that is subjected to the large pushing force is lapped in a large amount, leading to a variation in the lapping amount. If the pressing force varies, then elements subjected to a large pressing force may be damaged in the worst case. Moreover, since the elements are actually lapped in a certain amount in the surface finishing lapping process, variation in the element heights that is minimized in the previous element height forming lapping process may be increased again. According to the investigation conducted by the inventors of the present invention, the variation in the element height (MR height) after the surface finishing lapping process is performed is larger than the variation after the element height forming lapping process is performed by about 3 nm. An increase in the recording density of a magnetic head in the future requires a reduction in the element height, and therefore, an increase in the variation in the element height in the surface finishing lapping process makes it difficult to achieve higher recording density of a magnetic head. Variation in the pressing force may also increase variation in the dimension of recesses formed near the read and write element, i.e., PTR (Pole Tip Recession). For example, if a read element is retracted in a direction away from the air bearing surface relative to the substrate that is made of Al2O3/TiC, then the read element is away from a recording medium, and the desired reading property can not be achieved. Therefore, an increase in the variation in the PTR also leads to a degradation of yield.
Furthermore, if the pressing force varies, then the lapping plate itself, in turn, is subjected to large reaction force from the elements, at locations of the lapping plate (the concave portions) where a large pressing force is applied to the elements. The reaction force may cause fine scratches on the lapping plate, which may reduce the lifetime of the lapping plate because the surface finishing lapping process requires a lapping plate that is mirror finished with high precision.
However, it is actually difficult to maintain a uniform pushing force. FIG. 2 is a schematic view illustrating the relationship between force that is applied to a piston via air pressure (a product of differential pressure between the upper surface and the lower surface of a piston and a cross section thereof and a displacement of the piston. The pushing force of the pusher is in proportion to the displacement of the piston. The problem to be solved by the present invention will now be described more in detail with reference to FIG. 2. In the figure, force P, which is applied to the piston via air pressure, and displacement D of the piston are defined to be positive when they are directed downward in the figure (See FIG. 1).
As the pressure inside the cylinder is gradually increased, force P is increased gradually, and accordingly displacement D of the piston is also increased. The broken line shows an ideal case in which a linear relationship exists between force P and displacement D. In other words, when force P that corresponds to a desired pushing force, which is determined in advance, is given, a desired displacement X, and accordingly, a desired pushing force is always ensured. However, the relationship between force P and displacement D is actually non-linear because of friction between the piston and the cylinder. Specifically, when force P is gradually increased, the piston remains stationary for a while because of the friction, and when force P is further increased, the piston is moved (point A) and is stopped at point B. Thereafter, when the pusher is temporarily pushed upwards by the lapping plate or moves away from the lapping plate, for example, due to unevenness of the lapping plate, only displacement D temporarily fluctuates about point B while force P is kept constant. The fluctuating displacement that is caused by the upward pushing motion etc. is not in a linear relationship with the reaction force because of the friction between the piston and the cylinder. For this reason, when the upward pushing motion is ended and the initial state is recovered, displacement D of the piston does not always return to point B, but shifts, for example, to point C which is away from point B. If there is no friction between the piston and the cylinder, however, displacement D returns to displacement X after the upward pushing motion etc. is ended even if such motions temporarily occur.
As described above, the pushing force of a pusher is controlled by air pressure in the cylinder. However, a constant displacement, and accordingly, constant pushing force can not obtained even if constant air pressure is applied because of the non-linear relationship between force P and displacement D. In addition, even if constant air pressure is applied, displacement D fluctuates, and as a result, the pushing force also fluctuates. Therefore, it is difficult to obtain constant pushing force of the pusher, no matter how accurately the air pressure in the cylinder is controlled.